1. Field of the Invention
The present invention generally relates to the art of microelectronics, and more specifically to the fabrication of a multilayer substrate including a plurality of laminated layers of ceramic tape, and cavities formed in an external surface of the substrate.
2. Description of the Related Art
Multilayer substrates for hybrid microcircuits may be advantageously fabricated using a low temperature cofired ceramic process such as described in an article entitled "DEVELOPMENT OF A LOW TEMPERATURE COFIRED MULTILAYER CERAMIC TECHNOLOGY", by William Vitriol et al, ISHM Proceedings 1983, pp. 593-598. This technology utilizes dielectric sheets in the form of Low-Temperature-Cofired-Ceramic (LTCC) tape.
The LTCC tape sheets may be metallized to make a ground plane, signal plane, bonding plane, or the like, or formed with via holes and back filled with metallization to form interconnect layers. Individual sheets of tape are then stacked on each other, laminated together using a predetermined temperature and pressure, and then fired at a desired elevated temperature at which the material fuses or sinters.
A preferred LTCC, which is known in the art as "green tape", is commercially available from the DuPont Company under the product designation #851AT. The tape contains a material formulation which can be a mixture of glass and ceramic fillers which sinter at about 850.degree. C., and exhibits thermal expansion similar to alumina. The low-temperature processing permits the use of air fired resistors and precious metal thick film conductors such as gold, silver, or their alloys.
It is further desirable to form cavities in the upper external surface of the substrate to accommodate circuit components including chip capacitors and resistors, integrated circuits (ICs), diodes, transistors and voltage regulators, as described in an article entitled "The Use of Low-Temperature, Cofired Ceramic Technology for the Fabrication of High-Density, Hermetic, Multicavity Modules", by C. Sabo et al, in the Journal of the American Ceramic Society, Ceramic Substrates and Packages for Electronic Applications, Advances in Ceramics, Vol. 26, 1988, pp. 217-228. These cavities are conventionally formed using a computer-controlled punch. However, mechanical cavity punching is time consuming, wears the tools out quickly and is limited to regular shapes.
Conductive traces are conventionally formed on the external surfaces of multilayer LTCC substrates by thick film printing. However, the fired thickness of these traces is limited to approximately 10-15 micrometers using current technology, which provides insufficient electrical current and thermal conduction in some applications.